One or more embodiments of the present invention pertain to method and apparatus to control input to alternating current ([[xe2x80x9c]]AC[[xe2x80x9d]]) induction motors, and in particular, to control input to single-phase and three-phase AC induction motors.
As is well known, power is wasted when an alternating current ([[xe2x80x9c]]AC[[xe2x80x9d]]) induction motor is operated using a load below its rated load, or using a varying load.
U.S. Pat. Nos. 4,439,718 and 4,052,648 (the xe2x80x9cNola patentsxe2x80x9d) disclose methods for controlling power applied to an AC induction motor operating under varying loads based upon an evaluation of a displacement phase angle between voltage and current wave forms as they respectively reach zero value. The disclosed methods are effective only if the waveforms are purely sinusoidal, and, in actuality, such waveforms are rarely purely sinusoidal.
U.S. Pat. No. 4,379,258 (the xe2x80x9cSugimoto patentxe2x80x9d) discloses a power control circuit used to control voltage applied to an AC induction motor operating under varying load. The circuit detects supply power (equal to a product of an instantaneous value of voltage applied to the motor and current flowing through the motor, when the product is positive) and feedback power (equal to a product of an instantaneous value of voltage applied to the motor and current flowing through the motor, when the product is negative) flowing between a power supply and the motor. Further, Sugimoto discloses that the circuit controls the voltage applied to the motor such that a ratio between the supply power and the feedback power is maintained at a predetermined value. The disclosed circuit may be problematic because it is believed that the use of a predetermined value may need to be adjusted for various motors. In addition, the specific value stated by Sugimoto does not give the maximum savings possible.
In light of the above, there is a need for improved methods and apparatus to control power input to AC induction motors.
One or more embodiments of the present invention advantageously satisfy the above-identified need. In particular, one embodiment of the present invention is a method to control input to an alternating current ([[xe2x80x9c]]AC[[xe2x80x9d]]) induction motor from a power supply that comprises steps of: (a) determining a measure of reactive power ([[xe2x80x9c]]VAR[[xe2x80x9d]]) in two input lines to the motor during a time period; and (b) maintaining an SSR connected in series between the power supply and the motor in a non-conducting state for a subsequent time period, the length of which subsequent time period is determined by analyzing the measure of VAR.